Recently, as electronic devices become increasingly wireless and portable, a non-aqueous electrolyte battery with high capacity and high energy density is practically used as a power source for driving these electronic devices. However, this non-aqueous electrolyte secondary battery has a problem in that its capacity is reduced with the progression of charge/discharge cycles for the following various reasons, and particularly when it is exposed to a high-temperature environment, its capacity will be more remarkably reduced:                (1) A transition metal contained in a composite oxide constituting a cathode is dissolved in a non-aqueous electrolyte and deposited on an anode, resulting in the structural breakdown of the composite oxide in the cathode or an increase in interfacial resistance;        (2) The dissolved cathode transition metal continues to grow, thus causing micro-short circuits between the cathode and the anode;        (3) The cathode transition metal deposited on the anode acts as a catalyst promoting the decomposition of the non-aqueous electrolyte, thus causing gas generation within the battery;        (4) The SEI layer of the anode becomes thicker with the progression of charge/discharge cycles and prevents the migration of Li+; and        (5) The expansion and contraction of the anode active material causes slow breakdown of the SEI layer.        
Generally, the non-aqueous electrolyte secondary battery has a problem in that the electrode performance and efficiency are remarkably reduced, particularly at high temperature, for the following reasons: (1) the electrode resistance is increased due to a reaction between a cathode active material such as a lithium-containing metal oxide capable of absorbing and releasing lithium and/or lithium ions, and an electrolyte solution containing a carbonate solvent and a lithium salt; and (2) a solid electrolyte interface (SEI) layer formed on the surface of an anode active material capable of absorbing and releasing lithium and/or lithium ions is slowly broken at high temperature due to continuous charge/discharge cycles, while a poor SEI layer is produced from the carbonate solvent so as to accelerate irreversible reactions, including Li corrosion.
Meanwhile, in the non-aqueous electrolyte secondary battery, the cause of a problem in the battery safety upon overcharge is as follows: A cathode active material such as a lithium-containing metal oxide capable of absorbing and releasing lithium and/or lithium ions is changed into a thermally unstable material by lithium release upon overcharge. When the battery temperature reaches the critical temperature, the structural breakdown of the cathode active material which has been unstable occurs to release oxygen. The released oxygen and an electrolyte solvent, etc., cause an exothermic chain reaction, resulting in thermal runaway.
Generally, factors which can influence the safety of the battery upon overcharge may include: (1) exothermic heat caused by the oxidation of the electrolyte solution, and (2) exothermic heat caused by the structural breakdown of the cathode.
These exothermic heats occurring alone or in combination during the progression of overcharge result in an increase in the temperature within the battery, which leads to the fire or explosion of the battery, thus causing a problem in the battery safety upon overcharge.
Meanwhile, the fire and explosion phenomena of a lithium secondary battery, which are caused by thermal runaway, occur in the following cases: (1) local short circuits occur by external physical impacts (e.g., high temperature exposure by heating) in a state where the lithium secondary battery has been charged or overcharged; (2) the battery is exploded due to exothermic heat caused by a reaction between a flammable electrolyte solution and a cathode active material at high temperature; and (3) the combustion of the electrolyte solution is accelerated by oxygen generated from the electrodes (particularly, cathode).